Injection control method in an electrically-operated injection molding machine

ABSTRACT

An injection control method for an electrically-operated injection molding machine is provided, which is capable of sharply increasing the injection speed of a low-priced electric injection molding machine. A command pulse train is supplied to a servo circuit for an injection servomotor whereby substantial rotary motion of the injection servomotor is initially prevented, as a result of the fact that the torque limit value (Ts) is set at a value sufficient to establish equilibrium between the output torque of the injection servomotor and the resin pressure acting on the injection screw. The error between the target rotational position of the injection servomotor and an actual rotational position thereof then is allowed to increase until it reaches a feed stop positional error (Fs). The torque restriction is discontinued when the feed stop positional error Fs is reached, and a large drive current is caused to flow in the injection servomotor circuit in correspondence with the actual positional error which has now increased to the feed stop positional error Fs. Thus, the injection screw is initially driven by a great torque generated by the injection servomotor, so that the injection speed rises sharply.

TECHNICAL FIELD

The present invention relates to an injection control method for anelectrically-operated injection molding machine.

BACKGROUND ART

Depending on types of molded products, it sometimes is desirable to moresharply increase the injection speed at the beginning of an injectionprocess. To this end, in conventional hydraulic injection moldingmachines an accumulator is connected to a hydraulic injection cylinderfor supplying pressurized operating oil at a large flow rate to thecylinder. On the other hand, electrically-operated injection moldingmachines employ injection servomotors capable of producing a largeoutput with small inertia. However, injection molding machines equippedwith accumulators or injection motors capable of large output with smallinertia are costly.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an injection controlmethod for an electrically-operated injection molding machine, wherebythe injection speed may be sharply raised even while using a low-pricedelectric injection molding machine.

To achieve the above-mentioned object, according to the presentinvention, there is provided an injection control method which isapplied to an electrically-operated injection molding machine wherein aninjection servomotor driven in accordance with an error between a targetvalue and an actual value of an injection control parameter. Theinjection control method comprises the steps of: (a) preventingsubstantial rotational movement of the injection servomotor at aninitial stage of injection control, while the target value of theinjection control parameter increases; and (b) starting the rotationalmovement of the injection servomotor when the error between the targetvalue and the actual value of the injection control parameter reaches apredetermined value.

According to the method of the present invention, the injectionservomotor is capable of generating a great output torque uponinitiation of the rotational movement of the servomotor, even if theservomotor is other than a large output, small inertia type, because thetarget value of the injection control parameter associated with therotational movement of the injection servomotor is increased, whilesubstantial rotational movement of the injection servomotor isprevented, and the rotation of the injection servomotor is not starteduntil the error between the target value and the actual value of theinjection control parameter reaches a predetermined value. Accordingly,injection speed can rise rapidly even in a less costly injection moldingmachine, so that injection molding is carried out with a desiredinjection characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the operational principles of an injectioncontrol method according to an embodiment of the present invention;

FIG. 2 is a block diagram showing the essential parts of an injectionmolding machine to which the injection control method of FIG. 1 isapplied;

FIG. 3 is a block diagram the servo circuit of FIG. 2; and

FIG. 4 is a flowchart showing the injection control process as executedby the control unit of FIG. 2.

BEST MODE OF CARRYING OUT THE INVENTION

The operational principles of an injection control method according toan embodiment of the present invention will be explained with referenceto FIG. 1.

Conventionally in injection control methods wherein command pulses areperiodically distributed to a servo circuit to provide a targetrotational position, for an injection servomotor connected to the servocircuit for driving the same motor which position varies depending onthe pulse distribution level, the pulse distribution amount per pulsedistribution period is increased at an accelerating rate from the startof an injection process with lapse of time, and at the same time, actualrotational position of the servomotor is immediately caused to followthe target rotational position from the instant at which the injectionprocess is started, so that the axial movement of the injection screw isinitiated simultaneously with the beginning of the command pulse train.In order to rapidly increase the injection speed in such controlcircumstances the rotation rate of the injection servomotor must beincreased rapidly upon initiation of the axial movement of the injectionscrew. To this end, it is conventional to utilize high output, lowinertia injection servomotor.

The injection control method of the present invention contemplates theachievement of a rapid increase injection speed without the need forutilization of a large output, low inertia injection servomotor. Inother words, the invention/provides for the generation of a large motoroutput torque at the beginning of the axial movement of the injectionscrew even while using an ordinary servomotor. To this end, during theinitial stages of the supply of a command pulse train, substantialdriving of the injection servomotor, i.e., substantial axial movement ofthe injection screw is prevented. More specifically, a torque limitvalue Ts for restricting the output torque of the injection servomotoris set to a value sufficient to establish equilibrium between the motoroutput torque and the resin pressure acting on the screw, i.e., thepressure of the molten resin in the heating cylinder, which accommodatesthe injection screw, Thus the operation of the injection servomotor isfirst started only when the target rotational position of the injectionservomotor, which has been increased while substantial increase in theactual rotational position of the motor has been prevented, reaches apredetermined value (feed stop positional error Fs), i.e., only when theerror between the target rotational position and the actual rotationalposition reaches the predetermined value. More specifically, theapplication of the torque limit value Ts is discontinued only when thefeed stop positional error Fs has been reached. As a result, at the timethat the motor begins to rotate the injection servomotor is suppliedwith a large electric drive current determined by the actual positionalerror which has been increased to the value Fs. Thus, the injectionservomotor is able to generate a large output torque for axially drivingthe injection screw with a great driving force. As a consequence,injection speed rises rapidly.

Meanwhile, feed stop positional error Fs is a control parameter which isassociated with the conventional feed stop function of an injectionmolding machine. The feed stop function is used to temporarily stop thedistribution of command pulses and the same is rendered effective tostop such distribution instantaneously when the actual positional errorexceeds the value Fs and continues to be so effective until the actualpositional error again becomes less than the value Fs.

The electrically-operated injection molding machines to which theinjection control method according to an embodiment of the presentinvention is applied, comprise various operating sections (not shown)such as injection mechanisms screw rotation mechanisms mold-clampingmechanisms and eject mechanisms arranged so as to control the drive ofthese operating sections by means of a numerical control unit(hereinafter referred to as an NC unit), and a programmable machinecontroller (not shown). The injection mechanism includes an injectionservomotor 2 (FIG. 2), consisting of a D.C. servomotor, for instance,having a pulse coder 3 and which is operable to axially drive a screw 1disposed within a heating cylinder 4 through a rotary motion/linearmotion conversion mechanism (not shown).

Referring to FIG. 2, the NC unit 100 includes a central processing unitfor numerical control (hereinafter, referred to as NCCPU) 101 to whichis connected a ROM 101a storing therein a management program forglobally controlling the injection molding machine, and a RAM 101b fortemporary storage of data. Further, servo circuits for controlling thedrive of servomotors of the various operating sections are connected toNCCPU 101 through a servo interface 107 (only the servo circuitcorresponding to the injection servomotor 2 is shown by referencenumeral 106). The NC unit 100 further includes a central processing unitfor the programmable machine controller (hereinafter referred to asPMCCPU) 102 to which is connected a ROM 102a storing therein, e.g., asequence program for controlling the sequential operation of theinjection molding machine, and a RAM 102b for temporary storage of data.

Reference numeral 103 denotes a non-volatile shared RAM, consisting ofbubble memory, CMOS memory or the like, for storing therein an NCprogram and various molding condition parameters (injection speed,injection end screw position and the like); and reference numerals 104and 105 respectively denote an input circuit and an output circuit.Respective busses of the aforesaid elements 101-105 are connected to abus arbiter controller (hereinafter referred to as BAC) 108 forcontrolling the selection of busses to be enabled during certaininformation processing cycles of the NC unit 100. Further, a manual datainput device with a CRT display (hereinafter referred to as CRT/MDI) 110is connected to BAC 108 through an operator panel controller 109, sothat an operator is permitted to operate various operative keys ofCRT/MDI 110 including software keys and ten-key pad, so as to inputvarious control parameters including the molding condition parameters.

As shown in FIG. 3, servo circuit 106 for controlling the drive of theinjection servomotor 2 comprises an error register 200 arranged toreceive a command pulse train, which is then distributed by NCCPU 101through servo interface 107 and which is indicative of the targetservomotor rotational position, and a pulse train supplied from thepulse coder 3 and which is indicative of the actual servomotorrotational position. The output from error register 200 and which isindicative of actual positional error is converted into a velocityvoltage command by D/A converter 201, and a frequency at which thepulses delivered from pulse coder 3 are generated is converted into avoltage indicative of the actual rotational speed of the servomotor bymeans of a F/V converter 206, further the error (velocity error) betweenthe two voltages is amplified by an error amplifier 202 so as to providea torque command voltage. A torque limit circuit 203, receives on onehand the torque command voltage, and, on the other hand, a torque limitvalue delivered through circuit 105 from PMCCPU 102, so as to limit thetorque command value up to the torque limit value. Further, the errorbetween the torque command voltage after torque restriction and avoltage, which is detected by an appropriate means (not shown) and whichis indicative of actual motor drive current and therefore corresponds toactual motor output torque, is amplified by an error amplifier 204 and apower amplifier 205, and then the thus amplified error is applied toinjection servomotor 2 for control of the motor output torque.

Further, the injection molding machine comprises a feed stop functionfor temporarily stopping the pulse distribution when the stored value inerror register 200, i.e., the actual positional error, exceeds apredetermined value (hereinafter, referred to as feed stop positionalerror) set beforehand, until the actual positional once again becomesless than the feed stop positional error.

The operation of the injection molding machine described above is asfollows.

Prior to execution of an injection molding cycle by the injectionmolding machine, an operator operates a keyboard of the CRT/MDI 110 soas to input the feed stop positional error Fs and the torque limit valueTs, which values have been explained hereinabove with reference toFIG. 1. The torque limit value Ts is set to a value which permitsachievement of equilibrium between the output torque of injectionservomotor 2 and resin pressure at the initial stages of injectioncontrol. The feed stop positional error Fs is set to a value whichpermits the injection operation to begin with an injection speed whichrises sharply operation of the servo circuit 106, when the applicationof the torque limit value Ts to torque limit circuit 203 is discontinuedunder a condition where the actual positional error accumulated in errorregister 200 of servo circuit 106 has reached the value of Fs. In otherwords, the value Fs is set to a value which is different from, e.g.,larger than the ordinary feed stop positional error setting forprevention of overshoot and undershoot with respect to the targetservomotor rotational position during a conventionalacceleration/deceleration operations. Further, setting of variouscontrol parameters other than the values of Ts and Fs is carried out.Whereupon, the thus set control parameters are stored in respectivepredetermined address regions of shared RAM 103, under the control ofPMCCPU 102.

During operation of the injection molding machine, NCCPU 101 performsthe distribution of pulses to servo circuits of associated operatingsections of the machine through servo interface 107, and PMCCPU 102performs sequence control for associated operating sections, inaccordance with the NC program and various control parameters stored inshared RAM 103 and with the sequence program stored in ROM 102a. As aresult, the injection molding cycle, which consists of a series ofprocesses of mold-opening, mold-closing, mold-clamping, metering,injection, hold, ejection of a molded product, etc., is carried out inbasically the same manner as in conventional molding machines.

During the injection process, PMCCPU 102 executes the processing shownin FIG. 4.

At the start of the injection process, PMCCPU 102 reads out the torquelimit value Ts from shared RAM 103 and sets the same in the torque limitcircuit 203 of servo circuit 106, through BAC 108 and output circuit 105(step S1), PMCCPU 102 then delivers an injection start command to NCCPU101 through BAC 108 and shared RAM 103 (step S2). Upon receipt of thiscommand, NCCPU 101 starts pulse distribution a function which is to beexecuted by the NCCPU 101 at predetermined intervals during the cycle inaccordance with the molding conditions stored in shared RAM 103. As aresult, as shown in FIG. 1, a command pulse train is supplied from NCCPU101 to error register 200 of servo circuit 106. On the other hand, thetorque limit value Ts has been already set in step S2 as explained aboveand accordingly, the output torque of injection servo motor 2 is socontrolled as to be in balance with the resin pressure. As a result, thesubstantial rotary movement of injection servomotor 2 and substantialaxial movement of screw 1 are prevented. In this manner, the supply ofthe command pulse train is continued while rotation of the injectionservomotor 2 remains stopped, and thus, the accumulation of pulses inerror register 200 continues, and therefore the actual positional errorincreases with the elapse of time.

During this time period, NCCPU 101 writes, at predetermined intervalsduring the cycle, the actual positional error, and this value is readout by error register 200 through servo interface 107, and into sharedRAM 103. On the other hand, PMCCPU 102 determines whether or not theactual positional error written into shared RAM 103 has reached the feedstop positional error value Fs. When the actual positional error reachesthe value of Fs, PMCCPU 102 causes the torque limit value to change fromthe value Fs to a value corresponding to the maximum output torque ofinjection servomotor 2, for instance, to thereby discontinue the torquerestriction which has been applied to injection servomotor 2 (step S4).

When the torque restriction is discontinued torque limit circuit 203permits the supply of a large torque command voltage to error amplifier204, and such command voltage is delivered from error amplifier 202 at alarge value corresponding to a large actual positional error that isequal to the value Fs stored in error register 200 at that time. As aresult, in accordance with a control output from power amplifier 205corresponding to the large torque command voltage, a large drive currentis claused to flow to injection servomotor 2, so that injectionservomotor 2 which has been kept in an unrotating state while the torquerestriction was in effect, is now acused to rotate with a great amountof torque. As a result, screw is driven axially with a great drive forceby means of injection servomotor 2 through the rotary motion/linearmotion conversion mechanism. In other words, the injection speed risessharply.

Thereafter, when the actual positional error reaches the feed stoppositional error Fs during the injection operation by screw 1, the pulsedistribution for injection servomotor 2 is temporarily discontinued, andthe same is restarted only when the actual positional error againbecomes less than the value Fs. During the injection operation, PMCCPU102 determines whether or not the pulse distribution up to the injectionend screw position is completed i.e., whether the screw 1 has reachedits injection end position (step S5). If so, PMCCPU 102 completesprocessing of FIG. 4. And the conventional hold process is instuted.

The present invention is not limited to the aforementioned embodiment,but may be modified in various ways. For example, although, in thedescribed embodiment, the torque limit value Ts employed in the initialstage of the injection process is stored beforehand in shared RAM 103,the torque limit value Ts may instead be described in the sequenceprogram. Further, the present invention may be applied to an injectionmolding machine which is not equipped with a feed stop function althoughthe foregoing embodiment is applied to an injection molding machinewhich does have a feed stop function. In this case, a predeterminedpositional error corresponding to the feed stop positional error Fs isstored beforehand in shared RAM 103. Moreover, although in the abovedescribed embodiment the feed stop function of the injection moldingmachine is employed for obtaining a sharply rising injection speed, thisfeed stop function may be used in an ordinary manner for prevention ofovershoot and undershoot during acceleration/deceleration control, whilethe value of Fs for embodying the method of the invention isindependently set in shared RAM 103. In this case, the injection moldingmachine is provided with an ordinary feed stop function and the functionof sharply raising the injection speed is performed according to themethod of the present invention.

We claim:
 1. An injection control method for operation of anelectrically-operated injection molding machine having an injectionservomotor which is driven in accordance with an error between a targetvalue and an actual value of an injection control parameter, comprisingsteps of:(a) preventing substantial rotational movement of the injectionservomotor during an initial stage of injection control, while thetarget value of the injection control parameter increases; and (b)starting the rotational movement of the injection servomotor when theerror between the target value and the actual value of the injectioncontrol parameter reaches a predetermined value.
 2. An injection controlmethod according to claim 1, wherein said injection molding machine hasinjection screw that is driven by the injection servomotor, and whereina command pulse train is periodically distributed to a servo circuitoperatively connected to the injection servomotor so as to provide atarget value of a rotational position of the servomotor as saidinjection control parameter;wherein said step of preventing substantialmovement includes, in an initial stage of supply of the command pulsetrain, a step of setting a torque limit value for restricting the outputtorque of the injection servomotor to a value sufficient to establishequilibrium between the output torque of the injection servomotor andthe pressure of molten resin acting upon said injection screw to therebyrestrict the output torque of the injection servomotor; and wherein saidstep of starting the movement includes a step of discontinuing therestriction of the output torque of the injection servomotor when theerror between the target rotational position of the injection servomotorand an actual rotational position thereof increases to a predeterminedvalue.
 3. An injection control method according to claim 2, wherein thedistribution of said command pulse train is temporarily discontinuedwhen the error between the target rotational position and the actualrotational position of the injection servomotor reaches saidpredetermined value and until said error again becomes less than saidpredetermined value.